1. Field of the Invention
The present invention relates to the preparation of novel metal organic precursor compounds comprising at least one metal from Group IIb and at least one element from Group VIa of the Periodic Table. More specifically, these novel II/VI compounds are useful as single source metalloorganic tellurium, selenium, sulfur, zinc, cadmium or mercury containing precursors which are used in the preparation of semiconducting materials, e.g. as thin films, having precisely defined elemental ratios.
2. Description of the Related Art
Semiconducting thin-film alloys can be comprised of various combinations of elements from Group IIb(II) and VIa(VI) of the standard Periodic Table. These alloys are increasingly important due to their useful electronic, optoelectronic, magnetooptic and piezoelectric properties. These physical, chemical and electrical properties make the II/VI alloys of interest for a wide range of applications including, but not limited to, photovoltaic cells, infrared windows, light emitting diodes, and blue-green lasers.
In the conventional processing, the growth of II/VI thin films usually involves the use of two or more different volatile, toxic metal alkyl precursor compounds, i.e. one metalloorganic compound for each metallic element to be deposited. Each metalloorganic precursor has a different reactivity often causing undesirable pre-reactions to occur prior to mixing. Moreover, side reactions occur during processing which substantially limit the usefulness of the of metal alloy thin films which are produced.
These undesirable reactions can lead to difficulties in controlling the Group II/Group VI metal ratio in the resultant films, because one of the metal alkyl precursors is deposited before deposition of the desired metal alloy film on the substrate can occur. The toxicity of currently used II/VI precursor metalloorganic compounds also leads to considerable difficulties in storage, safety and environmental disposal. Moreover, the use of two or more metalloorganic compounds requires the extra capital expense of a second or third precursor compound line to the reactor in which the metal alloy is produced.
Considerable effort has been directed toward the synthesis of single source precursors to II-VI materials, with the majority of the work focused on the preparation of metal chalcogenolates M(ER).sub.2 (M=Zn,Cd,Hg; E=S,SE,Te R=organic). The thermal decomposition of Group IIb metal chalcogenolates to give binary ME materials has long been recognized. While these molecules would seem likely candidates for CVD processes, they are generally polymeric compounds. The suppression of polymer formation with the use of sterically demanding chalcogenolate ligands has been recently reported, and represents one possible way to enhance precursor volatility. However, these sterically saturated compounds show a tendency to reductively eliminate dichalcogenide to give elemental metal, rather than metal chalcogenide.
An alternative route to volatile precursors involves saturating the metal coordination sphere with strong neutral donor ligands. However, such ligands do not necessarily enhance compound volatility because the ligands tend to dissociate at temperatures lower than the temperatures at which the neutral complex sublimes. Moreover, this ligand dissociation process diminishes the utility of the precursor in metalloorganic chemical vapor deposition (MOCVD) processes.
Some general reports of the production of II/VI materials using separated metalloorganic compounds include, for example:
M. B. Hursthouse et al. (1991) Oganometallics, Vol. 10, pp 730-732, describes compounds of mixed alkyl tricarbonates of zinc and cadmium. These precursors are then used for deposition of semi conductors by metalloorganic chemical vapor deposition (MOCVD).
Y. Takaleaki et al. (1980) Journal of Crystal Growth, Vol. 50, p. 491, describes the preparation of precursors of cadmium or zinc dimethylthiophophinates for the production of cadmium sulfide or zinc sulfide.
J. O. Williams et al. (1992) Thin Solid Films Vol. 87, L1, describes the growth of highly ordered sulfide films using metal dimethylthiophosphinates.
A. Sounders et al. (1986) in "Ternary Multiary Compound" in Proceedings in the International Conference 7th, published in 1987, (see CA, 1988, Vol. 108, #66226H) describes the growth of sulfide films from zinc and cadmium thiocarbonates.
D. M. Frigo et al., (1989) Journal of Crystal Growth, Vol. 96, P. 989, describes sulfides of excellent crystallinity grown using bis(diethyldithiocarbonates).
M. Bochmann et al. (1989) Angew. Chem. International Edition in English Vol. 111, p. 414, discloses the preparation of low coordination number complexes of cadmium and zinc with sterically hindered thiols and selenols, such as 2,4,6-tri-tertiary-butylbenzene-thiol.
G. N. Pain et al. in Polyhedron, (1990) Vol. 9, #7, pp.921-929, discloses the preparation of organometallic cadmium, mercury and tellurium compounds which are used as precursors to metal alloys.
J. G. Brennan et al. in Chemistry of Materials, (1990) Vol. 2, pp.403-409, discloses the use of separate metalloorganic II/VI precursors useful in the preparation of metallic thin films.
S. M. Stuczynski et al. in Inorganic Chemistry, (1989), Vol. 28, #25, p.4431 and 4432, discloses the formation of metal-chalcogen bonds by the reaction of metal alkyl with silyl chalcogenides. However, they do not reach or suggest the preparation of all metals in a single precursor compound.
D. W. Kisher in Journal of Crystal Growth, (1989), Vol. 98, p.127-139, discusses the II/VI family of semiconductors alloys as obtained by organometallic vapor phase epitaxy (OMVPE), particularly in applied optoelectronics.
Phillip J. Bonasia et al. in Inorganic Chemistry, (1992), Vol. 31, pp.2508-2514, describes the preparation of homoleptic zinc, cadmium, and mercury tellurolates incorporating the bulky sitel anion. (sitel=TeSi(SiMe3)3).
Arnold (U.S. Pat. No. 5,157,136) disclose a single-source metalloorganic precursor used to produce II/VI materials. However, the present invention improves on all known precursors, including Arnold, by covalently binding a strong neutral donor atom to the chalcogenate functional group. This enhances compound volatility by decreasing polymer formation in the solid state, saturating the metal coordination sphere in the vapor phase and stabilizing gas phase species. Importantly, the present invention, unlike Arnold, eliminates the possibility of silicon incorporating impurities in the final solid state product. In the present invention, there is only one viable decomposition pathway because there is only one weak bond (C-E, E=S, Se, Te).
All of the references, patents, standards, etc. referenced in this application are incorporated herein by reference.
The problems in this art remain, i.e. the reactive precursors, undesirable pre-reactions, side reactions and non-volatility or low volatility. It would be extremely useful to have the metals of interest in a single, stable and volatile metalloorganic precursor compound so that the ratio of the metals deposited as an alloy upon decomposition can be more precisely controlled. The present invention provides such precursor compounds and processes to produce them.